trjhtr

This is an implementation of rotation (cyclic shift) of many-byte groups to the left. Rotate right is supported as negative amount. I would like to request, if someone would be willing, to review this code for correctness and performance. Both Python 2 and 3 need to be supported.

This code is part of Kaitai Struct framework. Current implementation can be found here. Code below is in PR #28, unmerged.

There is a test case for this feature here.

Support code:

if PY2:
 range = xrange
 def integers2bytes(ints):
 return bytes(bytearray(ints))
 def bytes2integers(data):
 return bytearray(data)
else:
 def integers2bytes(ints):
 return bytes(ints)
 def bytes2integers(data):
 return data

Main code:

class KaitaiStream(object):

 # formula taken from: http://stackoverflow.com/a/812039
 precomputed_single_rotations = amount: [(i << amount) & 0xff 

 @staticmethod
 def process_rotate_left(data, amount, group_size):
 amount = amount % (group_size * 8)
 if amount == 0:
 return data
 data_ints = bytes2integers(data)

 if group_size == 1:
 translate = KaitaiStream.precomputed_single_rotations[amount]
 return integers2bytes(translate[a] for a in data_ints)

 if len(data) % group_size != 0:
 raise Exception("data length must be a multiple of group size")

 if amount % 8 == 0:
 indices = [(i + amount//8) % group_size for i in range(group_size)]
 return integers2bytes(data_ints[i+k] for i in range(0,len(data),group_size) for k in indices)

 amount1 = amount % 8
 amount2 = 8 - amount1
 indices_pairs = [ ((i+amount//8) % group_size, (i+1+amount//8) % group_size) for i in range(group_size)]
 return integers2bytes((data_ints[i+k1] << amount1) & 0xff | (data_ints[i+k2] >> amount2) for i in range(0,len(data),group_size) for k1,k2 in indices_pairs)